Abstract

Marine cartilaginous fish retain a high concentration of urea to
maintain the plasma slightly hyperosmotic to the surrounding
seawater. In adult fish, urea is produced by hepatic and extrahepatic
ornithine urea cycles (OUCs). However, little is known about the urea
retention mechanism in developing cartilaginous fish embryos. In
order to address the question as to the mechanism of urea-based
osmoregulation in developing embryos, the present study examined
the gene expression profiles of OUC enzymes in oviparous
holocephalan elephant fish (Callorhinchus milii) embryos. We found
that the yolk sac membrane (YSM) makes an important contribution
to the ureosmotic strategy of the early embryonic period. The
expression of OUC enzyme genes was detectable in the embryonic
body from at least stage 28, and increased markedly during
development to hatching, which is most probably due to growth of the
liver. During the early developmental period, however, the expression
of OUC enzyme genes was not prominent in the embryonic body.
Meanwhile, we found that the mRNA expression of OUC enzymes
was detected in the extra-embryonic YSM; the mRNA expression of
cmcpsIII in the YSM was much higher than that in the embryonic
body during stages 28–31. Significant levels of enzyme activity and
the existence of mitochondrial-type cmgs1 transcripts in the YSM
supported the mRNA findings. We also found that the cmcpsIII
transcript is localized in the vascularized inner layer of the YSM.
Taken together, our findings demonstrate for the first time that the
YSM is involved in urea-based osmoregulation during the early to mid
phase of development in oviparous cartilaginous fish.